Conventionally, a number of methods have been proposed to detect and correct color misregistration of an image input apparatus that reads an object by means of a three-line sensor having three lines, such as R, G and B. Among them, Patent Literature 1 discloses a method for correcting even color misregistration caused by an optical axis misalignment of an optical system resulting from mechanical vibration or the like.
In the method of Patent Literature 1, reference patterns are provided in a read region of an object and edges of straight lines included in the reference patterns are read by a predetermined one sensor of three sensors that a three-line sensor has. And the read values are averaged to obtain an average value a which is regarded as a read value when the machine does not vibrate, and then, from the differences between the average value a and the each read value of the edges of the straight lines, a misregistration amount of the each read location on the edges of the straight lines is calculated out. Based on these misregistration amounts, misregistration amount of the each read locations in the read lines between the edges is determined, and, based on these misregistration amounts, the read output values of the sensors are corrected.